It is known that one may construct one or more thin film resonators ("TFR's") on a semiconductor wafer to form microwave antenna devices. In general, TFR antennas comprise a metal ground plane, a dielectric layer, and a top metal layer. The top metal layer (or transducer), the interface to the microwave transmission medium, is coupled to signal receiving and transmitting circuitry in any of the many manners known to those of ordinary skill in the art. One such coupling technique, known as "acoustical coupling" is disclosed in Weber U.S. Pat. No. 5,034,753 wherein the transducer is coupled to the electrical portion of the antenna system by means of piezoelectric resonators.
A large observed characteristic impedance between the metal layers of a TFR antenna reduces ohmic losses in the antenna in relation to the radiation resistance for providing the signal thus improving the signal gain of the antenna system and the value of the figure of merit, Q. It is therefore desirable to increase the characteristic impedance between the metal layers of a TFR antenna.
There is a marked degradation in signal gain for TFR antennas built upon semiconductor material such as silicon as opposed to gallium arsenide. This is a result of the fact that gallium arsenide is a semi-insulator while silicon is a semiconductor. Therefore, the dielectric losses for TFR antennas built upon a silicon substrate are larger than the ohmic losses for TFR antennas built upon a gallium arsenide substrate. In practice the increased losses severely restrict the usefulness of silicon, the most popular substrate material in the industry today for building microelectronic circuits, for fabricating TFR antennas. In view of the cost and manufacturing advantages of using a silicon substrate instead of gallium arsenide, it is desirable to provide a means for overcoming the inherently higher losses and signal degradation resulting from fabricating TFR antennas upon a silicon wafer.
An air bridge design is known for limiting capacitance of a micro-strip transmission line by providing a thin line of support posts approximately 5 microns high and spaced approximately 75 microns apart upon which a 5 micron wide transmission line is deposited. The transmission lines are intended to conduct signals on a line, but are not intended to radiate energy into the air. Thus, the object of the known bridge design is to isolate signals transmitted linearly on separate lines.